The present invention relates to measuring the liquid level or position of a gas/liquid interface along the length of a probe and, in particular, to a probe for measuring the temperature and gas/liquid interface position in a tank of cryogenic liquid.
In cryogenic liquid tanks (for example, propellant tanks for aerospace vehicles), it is important to know the temperature in various levels of the liquid and to know the level of the liquid within the tank. As the density of the liquid varies with temperature, this information is important to determine such values as the mass of the liquid in the tank.
Propellant tanks are usually more than 5 feet tall and typically in the range of 8 to 100 feet tall. One known device for the measurement of the temperatures in these tanks includes temperature sensors mounted on a metallic rake-like structure consisting of vertical and horizontal beams with temperature sensors mounted at various locations on the rake. Typical temperature sensors include silicon diodes, thermocouples, thermopiles, thermistors, and RTDs.
Another type of probe used for taking measurements of cryogenic liquid in a tank is disclosed in U.S. Pat. No. 6,431,750 to Haberbusch et al, which is incorporated by reference herein. The probe disclosed therein is a lightweight elongate flexible temperature probe including several temperature sensors embedded inside multiple layers of a thin film polyimide or polyester. In two of the layers, thin film copper is etched to form conductors to the sensors. The conductors are in abutting electrical contact with the sensors.
Known temperature probes, such as those described above, can be used to estimate the level of liquid the tank based on the temperatures sensed at various levels. However, in a partially empty tank, the cryogenic liquid may evaporate to form a layer of cryogenic gas above the liquid. Therefore, it can be difficult to ascertain the precise level of the liquid, since the cryogenic gas may have a temperature very close to that of the liquid.